A radio transmission device of a wireless communication system includes a power amplifier (hereafter, sometimes referred to as a “PA”) that amplifies the power of transmission signals. In the radio transmission device, the PA is usually operated near the saturation range of the PA in order to improve the power efficiency of the PA.
Furthermore, in recent years, in order to improve the spectral efficiency, a signal that is transmitted from the radio transmission device is sometimes a “multicarrier signal” that includes signals with multiple different carrier frequencies. Hereafter, a signal with each carrier frequency, which is included in a multicarrier signal, is sometimes referred to as a “carrier signal”. That is, a multicarrier signal includes multiple carrier signals. Examples of the multicarrier signal include an orthogonal frequency division multiplexing (OFDM) signal.
However, the peak-to-average power ratio (PAPR) of multicarrier signals tends to increase. Therefore, if a multicarrier signal is input to the PA that is operated near the saturation range, the waveform of the signal that is output from the PA is distorted due to the effect of the non-linear distortion of the PA.
Therefore, in the radio transmission device that transmits a multicarrier signal, “peak suppression” is conducted so that the peak amplitude (hereafter, sometimes simply referred to as the “peak”) of a multicarrier signal, which is to be input to the PA, is previously suppressed before it is input to the PA.
According to one of the techniques for peak suppression, as illustrated in FIG. 1, a suppression signal is applied to a multicarrier signal in synchronization with the timing (hereafter, sometimes referred to as the “peak timing”) in which the peak occurs in the amplitude waveform of the multicarrier signal. A subtraction amount that is a negative addition amount, i.e., a suppression amount, is determined from the difference between the peak value and the target value. Furthermore, an impulse response signal that has a signal component in the same frequency band as that of a transmission signal is used as a suppression signal. Thus, the peak of a multicarrier signal is suppressed to the target value, and therefore the linearity of signals that are output from the PA can be maintained. FIG. 1 is a diagram that illustrates an example of the peak suppression.
Examples of related-art are described in Japanese Laid-open Patent Publication No. 2014-027343, and in International Publication Pamphlet No. WO 2010/061914
Here, if the interval (hereafter, sometimes referred to as the “carrier interval”) between two adjacent carriers of a multicarrier signal is large, a small amplitude fluctuation of the multicarrier signal in the direction of the time axis is sharp, as illustrated in FIG. 2. Therefore, if the carrier interval is large, multiple peaks, e.g., eight peaks, that exceed the target value are sometimes detected within a short time range. FIG. 2 is a diagram that illustrates the problem.
With regard to the above, if impulse response signals are applied to the multicarrier signal in synchronization with the eight peaks, the peak is largely decreased with respect to the target value, i.e., the peak is suppressed too much. Therefore, it is preferable that, if multiple peaks that exceed the target value are detected within a short time range, the timing for applying an impulse response signal is determined to be one of the peak timings that correspond to the peaks. However, to determine the single optimum timing for applying an impulse response signal among multiple peak timings in accordance with various amplitude patterns of a multicarrier signal, processing is complicated, and the size of the circuit of the radio transmission device becomes large.